Clinical interpretation of androgen function in men relies predominantly on total testosterone (TT), yet a normal TT concentration does not exclude the symptomatic presentation of androgen deficiency. The Functional Androgen Axis (FOA) framework reframes androgen function not as a function of serum concentration but as the outcome of an interaction between two dimensions: signal intensity (Androgen Load, AL) and system response capacity (Receptor Capacity, RC). The functional result of this interaction is expressed as Androgen Signal Efficiency (ASE). AL aggregates the bioavailable serum signal, conversion and hormonal balance (estradiol, dihydrotestosterone), cumulative exposure time and exogenous potency. RC aggregates the physiological capacity to transduce that signal across metabolic, neuroendocrine (HPA), inflammatory, hematologic and local-utilisation domains, with receptor-level biology (androgen receptor expression, CAG polymorphism, coregulator availability) as its conceptual referent. The framework predicts that biological output is maximised not at maximal signal but at the point where load matches capacity (AL = RC). This paper provides the conceptual and mechanistic foundation of the FOA corpus. The document is presented as a working paper for methodological review.